A. Field of the Invention
The present invention relates to an automatic focus offset adjustment in a disc player, and more particularly to an automatic focus offset adjusting apparatus and method of a disc player for performing a reliable automatic controlling operation with respect to an abnormal condition of a disc such as a scratch or dust.
B. Description of the Prior Art
FIG. 1 is a functional block diagram showing a generally conventional compact disc player. Referring to FIG. 1, a pickup assembly 2 reads out data from a disc 1 to provide an electric signal. A RF signal generator 3 receives the electric signal supplied from pickup assembly 2 to produce a RF signal. The RF signal from RF signal generator 3 is amplified by a predetermined level in a RF amplifier 4, and is waveform-shaped in an analog waveform shaping part 5. Thereafter, the signal from analog waveform shaping part 5 is supplied to a digital signal processor 6 to be subjected to demodulating and decoding processing, thereby reproducing information recorded on disc 1.
Meanwhile, the output signal from pickup assembly 2 is provided to a focus error detector 7 and a track error detector 8. Focus error detector 7 detects a focus error signal FE from the signal supplied from pickup assembly 2 to provide the detected focus error signal to a servo controller 9. Track error detector 8 detects a track error signal TE from the signal supplied from pickup assembly 2 to provide the detected track error signal to servo controller 9.
Then, servo controller 9 receives focus error signal FE and track error signal TE respectively from focus error detector 7 and track error detector 8 to provide a focus control signal FC and a track control signal TC. A motor drive 10 receives focus control signal FC and track control signal TC to drive a slide motor 11, thereby transferring pickup assembly 2 up and down and side to side. Also, servo controller 9 controls a spindle motor 12 via motor driver 10 to rotate disc 1 at a predetermined speed.
FIG. 2 is a view showing a construction of the pickup assembly 2 of FIG. 1. As shown in FIG. 2, pickup assembly 2 has a laser diode 21 for radiating laser beam, and a collimator lens 22 for converting the diverging beam into parallel rays. In addition, a beam splitter 23 separates incident light and reflected light, and a quarter-wave plate 24 changes a polarized plane of the reflected light by 90 degrees. An objective lens 25 focuses the light, and a photodetector 26 converts the light from beam splitter 23 into an electric signal. Further, a focusing coil and a tracking coil (not shown) are disposed around objective lens 25.
Current flowing through the focusing coil applies a force in conformity with the Fleming's left hand law, and objective lens 25 attached to the coil is moved up and down to perform the focusing. Also, by current flowing through the track coil, objective lens 25 is moved side to side to perform the tracking.
To begin with, the laser beam produced from laser diode 21 is transformed into the parallel rays from the diverging beam by passing through collimator lens 22. At this time, a beam-splitting diffraction grating (not shown) is interposed between laser diode 21 and collimator lens 22. When the laser beam generated from laser diode 21 passes through the diffraction grating, three beams consisting of one main spot and two side spots are produced.
After this, the parallel rays from collimator lens 22 are focused onto objective lens 25 via beam splitter 23 and quarter-wave plate 24. Successively, objective lens 25 generates the beam spot to emit it to disc 1. The beam spot emitted from objective lens 25 is reflected from disc 1 to return to objective lens 25, and the reflected beam is to changed into parallel rays via objective lens 25. The parallel rays pass though quarter-wave plate 24 to advance toward beam splitter 23. Then, beam splitter 23 shifts the advancing direction of the parallel rays by as much as 90 degrees to permit the parallel rays to proceed toward photodetector 26.
FIG. 3 is a block diagram for illustrating a general principle of generating the RF signal, focus error signal FE and track error signal TE. Photodetector 26 includes four light-receiving devices A, B, C and D for receiving the main spot and two light-receiving devices E and F for receiving the side spots. Four light-receiving devices A, B, C and D convert the received light into electric signals and supply them to RF signal generator 3 and focus error detector 7. RF signal generator 3 sums the signals from light-receiving devices A, B, C and D to provide a sum signal A+B+C+D. The sum signal is used as the RF signal. The signals from light-receiving devices A, B, C and D are also provided as a difference signal (A+C)-(B+D) via focus error detector 7. The difference signal is used as focus error signal FE.
Light-receiving devices E and F receive the side spots to convert them into the electric signals. The signals from light-receiving devices E and F produce a difference signal E-F via track error detector 8. The difference signal E-F is used as track error signal TE. Thereafter, servo controller 9 receives focus error signal FE and track error signal TE to produce focus control signal FC and track control signal TC.
FIG. 4 is a block diagram for illustrating a conventional focus offset adjusting method. As is illustrated, servo controller 9 receives focus error signal FE to generate focus control signal FC. At this time, in order to provide accurate focus control signal FC, a variable resistor VR is manually regulated by a user to adjust a focus offset to generate focus control signal FC.
Furthermore, since the specific disc is adjusted only once in its fabricating line, the conventional method has the disadvantage of being incapable of obtaining the accurate focus offset value associated with the kind or state of the disc.